阅读理解. Since the beginning of human evolution(进化), men have migrated(迁移)across continents in search of food, shelter, safety, and comfortable weather. People still move for these reasons, but new reasons for human migration are arising, such as job relocation(重新安置) and overpopulation. Three million migrants are moving from poor countries to wealthier ones each year, and increasingly, their destination is a neighboring country in developing parts of the world. People are moving within the developing world for the same reasons as they migrate to wealthier nations. People from poor countries are going to less poor countries, fleeing wars and conflicts(冲突). They are also responding to population pressures because some countries are densely populated(人口密集), and they often have high population growth. Those people need to go somewhere else. There are three main reasons why people move. The basic categories and percentages are as follows, according to the Current Population Surveys (CPS): Family-related reasons account for 26.3%, including changes in marital status(婚姻情形), establishing a household and other family reasons; work-related reasons 16.2%, including job transfer, retirement, and other job-related reasons; housing-related reasons 51.6%, including new and better houses, better neighborhood, cheaper housing and other housing reasons; the remaining 5.9% of other reasons are attending college, the change of climate and health reasons. Americans have been migrating south and west for decades in search of better job opportunities and warmer climates. They have also been moving to places a little far from cities, in search of bigger yards and houses, lower crime rates and better schools. In 1950, nearly a fifth of the population lived in the nation's 20 largest cities. In 2006, it was about one in ten. That's why many American people say, 'Big Cities Shrink as People Move South, West.' Between March 2005 and March 2007, 73.4 million Americans moved. Fifty-six percent of these moves were within the same country. Twenty percent were between counties but in the same state. Nineteen percent were moves to a different state. Some families even went abroad.